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UC3845T Datasheet

(UC3842T - UC3845T) HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

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UC3842T UC3843T
® UC3844T UC3845T
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
. TRIMMED OSCILLATOR FOR PRECISE FRE-
QUENCY CONTROL
. OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
. CURRENT MODE OPERATION TO 500kHz
. AUTOMATIC FEED FORWARD COMPENSA-
TION
. LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
. INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT
. HIGH CURRENT TOTEM POLE OUTPUT
. UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
. LOW START-UP AND OPERATING CURRENT
DESCRIPTION
The UC384XT family of control ICs provides the
necessary features to implement off-line or DC to
DC fixed frequency current mode control schemes
with a minimal external parts count. Internally imple-
mented circuits include a trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
out featuring start-up current less than 0.5mA, a pre-
cision reference trimmed for accuracy at the error
amp input, logic to insure latched operation, a PWM
Minidip
SO8
comparator which also provides current limit control,
and a totem pole output stage designed to source
or sink high peak current. The output stage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltage lockout thresholds and maximum duty
cycle ranges. The UC3842T and UC3844T have
UVLO thresholds of 16V (on) and 10V (off), ideally
suited to off-line applications The corresponding
thresholds for the UC3843T and UC3845T are 8.5 V
and 7.9 V. The UC3842T and UC3843T can operate
to duty cycles approaching 100%. A range of zero to
< 50 % is obtained by the UC3844T and UC3845T by
the addition of an internal toggle flip flop which blanks
the output off every other clock cycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844T and UC3845T)
7
Vi
5
GROUND
4
RT/CT
VFB
COMP
CURRENT
SENSE
2
1
3
34V UVLO
S/R
5V
REF
2.50V
VREF GOOD
LOGIC
INTERNAL
BIAS
OSC
ERROR AMP.
+
-
2R
R
T
S
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
8 VREF
5V 50mA
6
OUTPUT
D95IN331
September 2001
1/15


STMicroelectronics Electronic Components Datasheet

UC3845T Datasheet

(UC3842T - UC3845T) HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

No Preview Available !

UC3842T - UC3843T - UC3844T - UC3845T
ABSOLUTE MAXIMUM RATINGS
Symbol
Parameter
Vi Supply Voltage (low impedance source)
Vi Supply Voltage (Ii < 30mA)
IO Output Current
EO Output Energy (capacitive load)
Analog Inputs (pins 2, 3)
Error Amplifier Output Sink Current
Ptot Power Dissipation at Tamb 25 °C (Minidip)
Ptot Power Dissipation at Tamb 25 °C (SO8)
Tstg Storage Temperature Range
TL Lead Temperature (soldering 10s)
* All voltages are with respect to pin 5, all currents are positive into the specified terminal.
PIN CONNECTION (top view)
Minidip/SO8
www.DataSheet4U.com
Value
30
Self Limiting
±1
5
– 0.3 to 5.5
10
1.25
800
– 65 to 150
300
Unit
V
A
µJ
V
mA
W
mW
°C
°C
COMP
VFB
ISENSE
RT/CT
18
27
36
45
D95IN332
VREF
Vi
OUTPUT
GROUND
PIN FUNCTIONS
No Function
Description
1 COMP This pin is the Error Amplifier output and is made available for loop compensation.
2 VFB This is the inverting input of the Error Amplifier. It is normally connected to the switching
power supply output through a resistor divider.
3 ISENSE A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
4 RT/CT The oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor RT to Vref and cpacitor CT to ground. Operation to 500kHz is possible.
5 GROUND This pin is the combined control circuitry and power ground.
6 OUTPUT This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced
and sunk by this pin.
7 VCC This pin is the positive supply of the control IC.
8 Vref This is the reference output. It provides charging current for capacitor CT through resistor RT.
ORDERING NUMBERS
SO8
UC3842TD
UC3843TD
UC3844TD
UC3845TD
Minidip
UC3842TN
UC3843TN
UC3844TN
UC3845TN
2/15


Part Number UC3845T
Description (UC3842T - UC3845T) HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
Maker STMicroelectronics
PDF Download

UC3845T Datasheet PDF





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